This invention relates generally to methods for increasing the corrosion resistance of metals, and specifically to methods of increasing the corrosion resistance of aluminum-based materials and aluminum-containing composites. The invention was made with government support under N00014-88-K-0034 awarded by the Office of Naval Research. The government has certain rights in the invention.
Aluminum-based materials and aluminum-containing composites are known to be relatively resistant to oxidation corrosion. However, such materials are susceptible to pitting corrosion when exposed to acids and halogens. For example, aluminums deteriorate rapidly when exposed to sea water. Even aluminums which are only exposed to the atmosphere will deteriorate with time because of pitting corrosion caused by acidic air pollutants and acid rain.
There are two commonly used methods of increasing the pitting corrosion-resistance of aluminums: anodizing and passivation with chromate solutions. Neither of these methods, however, is wholly satisfactory. Anodizing involves a complex and expensive multi-step procedure. Chromate passivation involves a less complex procedure but does not provide long-term corrosion protection. Chromate passivation, for example, does not provide sufficient pitting corrosion protection to allow aluminum-based materials to be used in marine environments. More importantly, chromates have recently been found to be carcinogenic and can be safely used only under rigidly controlled and expensive procedures. It is believed likely that chromate passivation procedures will be banned altogether within the near future for safety reasons.
It has been recently recommended to provide a corrosion-resistant surface layer on aluminums and other metals by incorporating cerium or other rare earth cations into the aluminum oxide surface film. However, there is no known method in the prior art of incorporating such cations into the surface film in a way which is both rapid (so that the process takes less than several days) and results in a uniformly corrosion-resistant surface.
Accordingly, there is a need for a non-complex method of increasing the corrosion resistance of aluminum-based materials which is superior to presently known methods and which does not require the handling of carcinogens.
There is a further need for a non-complex method of increasing the corrosion resistance of aluminum-based materials for long periods of time, even in severe atmospheres such as in sea water.